1. Field of the invention
The present invention relates to an imager employing CCD (charge-coupled device) and, more particularly, to an improvement in reducing a smear on a reproducing image.
2. Description of the Prior Art
Recently, many improvements have been made to an imager for use, for example, in a television camera, and one of which is the employment of a solid state CCD in place of the imaging tube, resulting in a compact size of the camera.
Such a solid state imager is defined by a plurality of photoelectric transducers aligned in two directions orthogonal to each other. When an image is formed on the solid state imager, each photoelectric transducers stores a charge which is in relation to the intensity of the impinged light thereon, thereby forming a charge pattern of one field. The charge pattern is transferred to a register, from which a charge signal is read out serially. When the image formed on the solid state imager has a bright portion, such as a bright spot light, an extra charge will be added, during the transfer of the charged pattern, to the photoelectric transducers on which the bright spot light is traced. Such an extra charge will result in an unpleasant smear in the reproduced image.
Conventionally, there have been proposed a number of arrangements which can reduce the smear. Such an arrangement is disclosed, for example, in U.S. Pat. No. 4,010,319 to Peter Alan Levine, or in Japanese Patent Laid-Open application No. 57-17276 to Michio Masuda et al.
One arrangement according to the prior art CCD imager is shown in FIG. 1, and which includes a frame transfer type CCD 1 having first register 11, second register 12, and readout resistor 13. In first register 11 defining an imaging area, photoelectric transducers P' in the bottom row are masked and the other photoelectric transducers P are provided to receive light image. Second register 12 defining a storage area and readout register 13 are also masked. When the clock pulse is applied to CCD 1, an image integration period (for example, 16.1 milliseconds) and an image transfer period (for example, 0.5 millisecond) are repeated alternately. It is assumed that a scene having one bright circle spot is projected on first register 11.
Before the first integration period starts, a "field" with no image information appears in first register 11 as if a "field" is pulled down across first register 11 in a blind pull down manner. First register 11 receives light even during the "field" is pulled down. Accordingly, the photoelectric transducers P' in the last row H' carry a signal obtained by the quick scan of the new field vertically across the light receiving area. When the image formed on the field has no outstanding bright portion, the signal in the last row H' is so low that it can be disregarded. On the contrary, when the image on the field has a bright portion, such as a bright spot, a photoelectric trandsducer will be integrated to a small degree even during the field scans across that bright spot. Thus, in such a case, the charge distribution along line Lm shown in FIG. 3a which extends through the masked last row H' has a small mount as indicated in graph G.sub.Lm. Since such a small mount appears on, and added to, every horizontal line a vertical smear will appear in that field. Thus, the signal shown in graph G.sub.Lm is called a smear signal.
During the first integration period, photoelectric transducers P are integrated, each charged to a level relative to the brightness of the received light. Accordingly, the received image is changed to a charge pattern on photoelectric transducers P. The charge distribution along line L1 shown in FIG. 3a which intersects the spot image is indicated in graph G.sub.L1.
Then, in the first transfer period, which corresponds to the vertical blanking period of the commercial television system, the charge signals which have accumulated one "field" are transferred, in parallel, in the column direction from register 11 to resistor 12, such that the charge in the photoelectric transducers P in the first row from the top are transferred to those in the second row, and so on. During the transfer period, a next new field appears in first register 11 as the same manner described above in the blind pull down manner.
Then, in the second integration period, the charge pattern is formed in the same manner described above, and at the same time, the signals stored in the second register 12 are readout through the readout register 13 serially such that the first readout line signal, which is the smear signal carried in the last row H' in photoelectric transducers P', is transferred and stored in line memory 2. After the readout of the smear signal, switch S changes its connection from the condition shown in FIG. 1 to such that terminal Ta is connected to terminal Tc and terminal Td is connected to terminal Tf. Accordingly, the readout signal through terminals Ta and Tc and the smear signal from line memory 2 are transferred at the same time to subtracter 3 at which readout signal is subtracted by smear signal. The subtracted result, such as indicated in graph G.sub.L1-Lm, is produced from the subtracter 3. When the subtraction is carried out for each horizontal line signal, an image without a smear, such as shown in FIG. 3b, can be reproduced.
According to the prior art CCD imager described above, a problem is created when the imager is provided with an overflow drain, bus, for the reduction of the blooming. FIG. 2 diagrammatically illustrates an arrangement of overflow drain 14 interleaving the aligned photoelectric transducers P. When a very bright image is impinged, photoelectric transducers generate much more charge signal than can be stored at that location. The excess charge tends to spread to the adjacent locations along the chargecoupled channel, resulting as "blooming" of the image. But when overflow drain 14 is provided, the excess charge, which is above a predetermined level Vmax, flows through the overflow drain 14, thereby eliminating undesirable blooming.
In the case where overflow drain 14, or the like which reduces the blooming, is employed in combination with the above described smear reducing arrangement, a problem arises as explained below.
When a very bright image, such as a very bright spot light as shown in FIG. 4a, impinges on the CCD array, the charge distribution along line L1 would have a flat top, restricting the maximum voltage to Vmax, as indicated in graph G.sub.L1. When the smear signal shown in graph G.sub.Lm is subtracted from the line signal along line L1, the flat top will be recessed, as shown in graph G.sub.L1-Lm, providing a dark portion at the center of the spot, as illustrated in FIG. 4b.